Having the same problem. I'm using a Flagello+Phago+Secro organism but whenever I try to split it the offspring dies right away no matter how I set split mass/split ratio/nutrient priority. I even searched for a YouTube playthrough of this stage but the video showed pretty much nothing.

I made a four cell swimmer flagg+phago+keratino+secro. But it's offspring always died immediately. I tried making it attach to the mother cell. That didn't work either. It got really wobbly.I tried messing with the skeleton thing. Didn't change a thing... Then I gave up.